168 Dr. Waller's additional Observations on Hail, 



ally adding a drop of distilled water, so as to prevent their 

 complete desiccation. At the end of this time the group of 

 vegetation had but slightly increased, but around were several 

 other groups of vegetations of the same nature which were not 

 perceived at the commencement of the examination. Their 

 examination, which hitherto had been made nearly every day, 

 was then abandoned as presenting no further object. 



As it is possible to distinguish snow of different formations, 

 its minute examination made simultaneously by different ob- 

 servers over a considerable extent of country, would enable us 

 in all probability to determine several interesting points con- 

 nected with the formation of snow. By their aid we might 

 ascertain to what distance in certain cases the same crystalline 

 form is preserved, and in what manner this is effected ; whether 

 by a simultaneous formation, or by a gradual propagation from 

 one point to another ; also at what rate of speed this is effected. 



The frequent transformation of the vapoui's of water from 

 the gasiform to the nebular state, commencing from a small 

 cloud at the horizon and rapidly extending to the whole visible 

 sky, bears a close analogy to the action of crystallization of 

 bodies in solution in a liquid. In both cases a molecular ac- 

 tion commencing from a circumscribed point is gradually 

 propagated to the entire mass, when at a proper degree of 

 saturation in the liquid solution the crystals formed are all 

 alike. In the meteorological phajnomenon, as it is a liquid 

 that falls, the crystalline action cannot be observed ; but the 

 frequent production of hail renders it not unlikely that the 

 clouds may sometimes be composed of solid particles. 



From their connection with the preceding remarks I will 

 mention the following experiments : — 



Exp, 1. A saturated solution of sulphate of soda covered 

 with a thin coating of oil was allowed to cool, and remained 

 at rest for several hours. On a glass rod being brought in 

 contact with the surface, the well-known plia?nomenon of the 

 solidification of the solution ensued. The speed at which this 

 solidification travelled from one extremity of the tube to the 

 other was at about the rate of one foot in forty seconds. The 

 crystalline particles thus formed consisted of long aciculai 

 about jij^Qjjth of an inch in thickness. 



Exp. 2. In a similar solution there were deposited, previous 

 to its solidification, beautiful transparent, hard crystals, which 

 are known to differ from Glauber's salts by their containing 

 only eight equivalents of water of crystallization to one of salt. 

 After the solidification had taken place the crystals were found 

 to be changed to a milk-white colour, although they still re- 

 tained the same shape. Under the microscope they were 



